1. Field of the Invention
The invention relates to the field of MOS buffer circuits such as inverter circuits.
2. Prior Art
In MOS circuitry there is often a need for controlling one signal with another such as controlling the output of a circuit with an input signal or for buffering a signal. In such circuits it is often desirable or necessary to have the output voltage equal to or very close to the source of potential used to power the circuit. More specifically, on an MOS chip, it is necessary in some cases to generate signals such as the inverse or complement of a timing signal where the inverse of complement is of the same magnitude as the voltage used to power the chip. This is particularly important where the inverted signal is utilized to drive other MOS devices on the chip.
Most often the output signal from a buffer or inverter is controlled by an output MOS device such as a field effect transistor. These devices generally have a voltage drop approximately equal to a threshold voltage between their source and drain terminals. Even where the potential applied to the gate of the output MOS device is equal to the potential applied to this device, this voltage drop exists. It is necessary to apply a potential to the gate of the output MOS device which is greater than the source of potential applied to the terminals of the device in order to avoid the undesirable threshold voltage drop.
Bootstrapping is a well known prior art technique for generating a voltage in an MOS circuit which is greater than the potential applied to the circuit. It is often used where it is necessary to avoid the effects of the threshold drop of an MOS device. Most often the bootstrapping utilizes a capacitor such as Miller capacitance to raise the potential on the gate of an MOS device to a potential greater than the potential across the source and drain of the device so that the effects of the threshold drop may be eliminated. Typically, in order to bootstrap the output transistor of a circuit, current must be supplied to that transistor when it is not supplying an output signal. Often the output transistor is large so it may drive other circuits and the standby power consumed by this device causes the circuit to be inefficient.
As will be seen, the present circuit utilizes a plurality of bootstrap circuits and a feedback path to provide a control circuit which is disclosed in the form of an inverter. The output signal is equal to the voltage used to power the circuit. The output transistor does not draw any current, except that supplied to the output.